Significantly higher-density recording has been demanded in connection with the development of magnetic recording mediums in recent years. Attempts to smooth the surface of the magnetic layer have been made as a possible means for attaining enhanced high-density recording.
For example, the surface of the magnetic layer is smoothed by incorporation of finely comminuted ferromagnetic powder or by pressing the surface thereof when the magnetic layer is a coating type magnetic layer composed of ferromagnetic powder dispersed in a binder resin. Additionally, in magnetic recording mediums having a magnetic layer composed of a ferromagnetic metallic thin film and designed for use as a high-density recording layer, the surface of the magnetic layer has a surface even smoother than that of said coating type magnetic layer.
When the surface of the magnetic layer is smoothed, a coefficient of friction of the magnetic layer with each part in a recording and reproducing apparatus is increased during the running of magnetic recording mediums. As a result, there is a greater risk that the surface of the magnetic layer will be damaged, or that the magnetic layer may be peeled off from the non-magnetic support.
Furthermore, given the popularity of floppy disk driving type devices, VTR, personal computers and word processors, magnetic recording mediums in recent years have been used over a wide range of conditions of from low temperature such as -10.degree. C. to high temperature such as 40.degree. C. under high-humidity such as 80% RH environmental conditions.
Accordingly, magnetic recording mediums are demanded which have good running durability under wide variety of environmental conditions and which possess stable characteristics which do not undergo significant change in response to environmental conditions.
To solve the above described problems, prior attempts have been made to contain lubricants such as fatty acids, fatty acid esters, hydrocarbons and silicone compounds in the magnetic layer or the surface thereof.
However, sufficient durability cannot be imparted to magnetic recording mediums capable of coping with recent high-density recording by these conventional lubricants.
The application of fluorinated oils such as perfluoropolyethers to ferromagnetic metallic thin film type magnetic recording mediums such as typically high-density recording video tapes has been examined. For example, perfluoropolyethers have been used which have terminals modified by addition of polar groups as described in U.S. Pat. Nos. 4,267,238 and 4,268,556, West German Patent 3,000,583 and JP-B-60-10368 (the term "JP-B" as used herein means an "examined Japanese patent publication"). The perfluoropolyethers having terminal polar groups are excellent in fixation to the surfaces of the magnetic layers of the magnetic recording mediums.
Further, attempts have been made to provide good lubricity as well as good fixation to the surface of lower magnetic layer by using a combination of a perfluoropolyether having a polar group with a perfluoropolyether having no polar group [see, JP-A-61-113126 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and U.S. Pat. No. 4,267,238]. The application of these perfluoropolyethers has an effect of enhancing durability over a wide range of from low temperature to high temperature conditions in comparison with conventional hydrocarbon lubricants. However, a problem is encountered in that a coefficient of friction is increased when the recording medium repeatedly run under low humidity (i.e., 10% RH) conditions.
The present invention is intended to solve such problems associated with the prior art as mentioned above.